CILIARY MECHANISMS OF LAMELLIBRANCHS 679 
Palps. The ciliation of the thick, heavy palps presents the 
usual features. In figure 48 a part of the anterior palp of the left 
side has been removed (at p) and exposes a part of the folded 
surface of the posterior palp, and the primary course across folds 
toward the mouth. The ventral edge, with its backwardly 
directed current, is very narrow. The outer faces, or those with- 
out folds, are ciliated, and direct material backward to the tips. 
Mantle. Posteriorly and below the line of gill attachment 
(ga), material is moved to a line lying parallel with the mantle 
edge, and then along this to a point y, between which and the 
point 2, it is carried over the edge. Between y and z on the edge, 
there is a feeble ciliation directed outward over the edge. Pos- 
terior to the adductor muscle (a) and above the line of gill attach- 
ment, there is a feeble ciliation upward and backward, over the 
edge above the point z. There is here also a strong current of 
water from the gills, which takes the same course, and aids in 
sweeping outward feces from the rectum (r). The mantle and 
the greater part of the visceral mass wall are fused on the left 
side. On the right, there is a space (e) anterior to the adductor, 
where mantle and epibranchial chambers communicate. The 
mantle wall here, and also that of the visceral mass, possess a 
ciliation directed upward. 
Figure 49 represents the mantle ciliation of O. lurida of the 
Pacific. At the point x is a narrow line over which collections 
pass from the mantle edge. 
Pholadidea penita Conrad 
Specimens were taken from San Diego Bay, California. The 
form is a true borer; it possesses an unpaired plate dorsal to the 
hinge. 
Gills. Cilia currents on all lamellae are to the free edges of the 
demibranchs and forward (fig. 50). There is a current forward 
in the angle between demibranchs. 
Palps. The lateral extension of these organs is great. The 
ventral margins are well defined, and those on the dorsal edges 
are exceptionally wide. The currents of the apposed faces are 
